The present invention relates to a semiconductor device, in particular, a nitride semiconductor device functioning as a blue laser, a field effect transistor for high-speed operation, and the like, a method for fabricating such a semiconductor device, and a method for fabricating a semiconductor substrate used in a semiconductor device
There are conventionally known lasers and field effect transistors that use as the active region thereof a compound semiconductor layer made of a nitride semiconductor, in particular, a group III nitride typified by gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN). In other words, many techniques utilizing features of nitride semiconductors already exist, including forming lasers (for example, blue lasers) that emit short-wavelength light utilizing the broad band gap of nitride semiconductors, and forming field effect transistors achieving high-speed operation utilizing the high mobility (traveling velocity) of carriers in nitride semiconductors.
FIG. 12 is a cross-sectional view of a conventional semiconductor device as a semiconductor laser using a nitride semiconductor. In FIG. 12, hatching of the cross section is omitted for clarification of the structure of defects in crystals. Referring to FIG. 12, on a substrate 101 made of n-type GaN, sequentially grown by epitaxy are an n-type GaN layer 111, an n-type AlGaN cladding layer 112, an n-type GaN optical guide layer 113, an undoped GaN active layer 114, a p-type GaN optical guide layer 115, a first p-type AlGaN cladding layer 116, a current narrowing layer 117 having an opening, a second p-type AlGaN cladding layer 118, and a p-type GaN contact layer 119 in this order. An n-side electrode 120 is formed on the bottom surface of the substrate 101, and a p-side electrode 121 is formed on the top surface of the p-type GaN contact layer 119.
The semiconductor device with the above construction includes the undoped GaN active layer 114 made of a nitride semiconductor. Therefore, by applying a voltage through the n-side electrode 120 and the p-side electrode 121, the semiconductor device can be used as a semiconductor laser device that oscillates blue light at an active region 114a of the undoped GaN active layer 114 located below the opening of the current narrowing layer 117.
The above conventional semiconductor device has a problem as follows. The substrate 101 intrinsically includes streaky lattice defects D (in particular, dislocations) extending vertically. Note that the substrate 101 also includes lattice defects such as dislocations extending in parallel with or in directions declined from the substrate plane. These dislocations have little relation to the cause of the problem to be described hereinafter, and thus are not shown in the figures. With the sequential epitaxial growth of the n-type GaN contact layer 111, the n-type AlGaN cladding layer 112, . . . on the substrate 101, the lattice defects extend upward, reaching the active region 114a of the undoped GaN active layer 114 located below the opening of the current narrowing layer 117.
In the semiconductor laser device, for laser oscillation, a high current must be applied to the active region 114a to generate an inversion state in the active region 114a. However, when such a high current is applied to the active region 114a that includes a number of lattice defects, deterioration of the laser oscillation function may possibly develop from the positions of the lattice defects and, as a result, the life and reliability of the semiconductor laser may be significantly reduced.
The above problem due to the existence of defects may arise, not only in semiconductor laser devices, but also in other semiconductor devices such as high-speed field effect transistors and Schottky diodes. For example, if a number of lattice defects exist in the channel region below the gate of a field effect transistor, the mobility of carriers decreases. This may possibly deteriorate the performance of the transistor.
As described above, a semiconductor device may possibly be deteriorated in performance due to lattice defects existing in the active region (carrier traveling region) thereof, such as the active layer in the case of a semiconductor laser device and the channel region in the case of a transistor.
An object of the present invention is providing a semiconductor device with high reliability and high performance capable of reducing the number of lattice defects in the active region thereof, a method for fabricating such a semiconductor device, and a method for fabricating a semiconductor substrate used in a semiconductor device.
The semiconductor device of the present invention includes: a substrate having a first semiconductor layer; at least one convex portion formed in the first semiconductor layer, the convex portion having a top surface and a side face intersecting with the top surface; a coat layer formed to cover at least part of the top surface and leave open at least part of the side face of the convex portion of the first semiconductor layer, the coat layer having a function of suppressing epitaxial growth of a semiconductor on the first semiconductor layer; and a second semiconductor layer formed on the first semiconductor layer by epitaxial growth, wherein a region of the second semiconductor layer located above the convex portion operates as an active region.
With the above construction, the following effects are obtained. A semiconductor crystal epitaxially growing from the side face of the convex portion of the first semiconductor layer is deposited in a direction roughly normal to the side face. During this crystal growth, lattice defects exposed on the side face of the first semiconductor layer are incorporated in the crystal constituting the second semiconductor layer, extending in the second semiconductor layer in a direction roughly normal to the side face of the convex portion, that is, in a direction away from the convex portion. Therefore, the lattice defects in the first semiconductor layer deposited by side-direction growth of the crystal epitaxially grown from the side face of the convex portion, such as the portion located above the coat layer. Thus, the region of the second semiconductor layer located above the convex porion constitutes a low defect region, and a semiconductor device having its active region in this low defect region can exhibit good characteristics. For example, when the device is a semiconductor laser device, the light emitting characteristics are suppressed from deteriorating. When the device is a field effect transistor, the carrier traveling characteristics are improved.
The coat layer may cover a portion of the semiconductor layer other than the top surface of the convex portion. This reduces the possibility of propagation of the lattice defects in the first semiconductor layer into the second semiconductor layer, and thus it is possible to provide a semiconductor device in which the defect density of the second semiconductor layer is lower.
At least two convex portions may be formed, and the coat layer may also cover a bottom surface of a concave portion formed between the at least two convex portions. Lattice defects extending from the side faces of the two convex portions sandwiching the concave portion (side faces of the concave portion) in a direction roughly normal to the side faces in the second semiconductor layer concentrate near the center of the concave portion and are united into roughly one streak, which then extends upward. Contrarily, no lattice defects propagate from the bottom surface of the concave portion into the second semiconductor layer. This greatly reduces the defect region in the second semiconductor layer, and thus further reduces the defect density of the entire second semiconductor layer.
A plurality of convex portions may be formed, and the top surfaces of the convex portions may constitute a stripe pattern. Thus, a semiconductor device having a strip structure suitable for a semiconductor laser device is provided.
The coat layer may also have a stripe pattern.
The coat layer may be made of a material selected from an oxide, a nitride, and a metal. In particular, the coat layer is preferably made of a material selected from silicon oxide, silicon nitride, and tungsten. More preferably, the coat layer is made of aluminum oxide.
The first and second semiconductor layers are preferably made of a Group III nitride.
The semiconductor device may further include a third semiconductor layer formed on the top surface of the convex portion of the first semiconductor layer, and the coat layer may be formed by oxidizing a surface portion of the third semiconductor layer. By appropriately selecting the material constituting the third semiconductor layer, the adhesion between the coat layer and the first semiconductor layer can be improved. This makes it possible to improve the yield at the fabrication of the semiconductor device.
The third semiconductor layer may be made of an Al-containing semiconductor, and the coat layer may be made of an oxide containing Al as a constituent element. This improves the adhesion between the coat layer and the first semiconductor layer, and thus can improve the yield at the fabrication of the semiconductor device
The above third semiconductor layer may be made of AlxGa1xe2x88x92xAsyN1xe2x88x92y (0 less than xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) such as AlAs.
The semiconductor device may further include an Al-containing underlying semiconductor layer immediately below the first semiconductor layer, and the convex portion of the first semiconductor layer may be a mesa-shaped convex portion isolated on the underlying semiconductor layer.
In the case of the above construction, the coat layer may also cover a bottom surface region of the underlying semiconductor layer that is not covered with the convex portion, and a surface portion of the underlying semiconductor layer in the bottom surface region may be oxidized. This further reduces the defect density of the second semiconductor layer.
The underlying semiconductor layer may be made of a material selected from the preferable materials exemplified for the third semiconductor layer.
The first method for fabricating a semiconductor device of the present invention includes the steps of: (a) forming a coat layer on a first semiconductor layer of a substrate, the coat layer being made of a material having a function of suppressing epitaxial growth of a semiconductor on the first semiconductor layer; (b) etching the coat layer and the first semiconductor layer to form in the first semiconductor layer at least one convex portion having a top surface and a side face intersecting with the top surface while forming a top epitaxial mask made of the coat layer on the at least one convex portion; (c) forming a second semiconductor layer on the first semiconductor layer by epitaxial growth after the step (b); and (d) forming a semiconductor element operating using a region of the second semiconductor layer located above the convex portion as an active region.
By the above method, the following effects are obtained. In the step (c), a semiconductor crystal epitaxially growing from the side face of the convex portion of the first semiconductor layer is deposited in a direction roughly normal to the side face. During this crystal growth, lattice defects exposed on the side face of the first semiconductor layer are incorporated in the crystal constituting the second semiconductor layer in a direction roughly normal to the side face of the convex portion, that is, in a direction away from the convex portion. Therefore, the lattice defects in the first semiconductor layer hardly extend in the portion of the second semiconductor layer deposited by side-direction growth of the crystal epitaxially grown from the side face of the convex portion, such as the portion located above the top epitaxial mask. Thus, the region of the second semiconductor layer located above the convex portion constitutes a low defect region, and a semiconductor device having its active region in this low defect region can exhibit good characteristics. For example, it is possible to provide a laser device where the light emitting characteristics are less deteriorated and a field effect transistor having superior carrier traveling characteristics.
In the step (b), at least two convex portions may be formed, and the method may further include the step of forming a bottom epitaxial mask on a bottom surface of a concave portion sandwiched by the two convex portions after the step (b) and before the step (c). This makes it possible to provide a semiconductor device in which the defect density of the second semiconductor layer is lower.
The method may further includes the steps of: forming an etching mask film after the step (a) and before the step (b); and patterning the etching mask film to form an etching mask after the step (a) and before the step (b), and in the step (b), the coat layer and the first semiconductor layer may be etched using the etching mask.
In the step (a), a film made of a material capable of selectively etching the first semiconductor layer may be formed as the coat layer. In the step (b), the coat layer may be patterned to form an etching mask, and then the first semiconductor layer may be etched using the etching mask. In the step (c), the second semiconductor layer may be epitaxially grown using the etching mask as an epitaxial mask.
In the step (a), a SiO2 film may be formed as the coat layer. This facilitates the formation of the epitaxial mask.
The second method for fabricating a semiconductor device of the present invention includes the steps of: (a) forming an etching mask on a first semiconductor layer of a substrate; (b) etching the first semiconductor layer using the etching mask to form in the first semiconductor layer at least one convex portion having a top surface having a size smaller than the etching mask and a side face intersecting with the top surface; (c) forming a second semiconductor layer by epitaxial growth on the first semiconductor layer after the step (b); and (d) forming a semiconductor element operating using a region of the second semiconductor layer ranging from the side face of the convex portion to part of a bottom surface of a concave portion as an active region.
By the above method, as in the first method, it is possible to reduce the number of lattice defects in the portion of the second semiconductor layer deposited by side-direction growth of the crystal epitaxially grown from the side face of the convex portion of the first semiconductor layer. Lattice defects propagating from the top surface of the convex portion of the first semiconductor layer into the second semiconductor layer extend in a direction roughly normal to the top surface of the convex portion. In this relation, however, the area of the top surface of the convex portion is reduced. The defect density is small in the region of the second semiconductor layer ranging from the side face of the convex portion to part of the bottom surface of the concave portion. Thus, a semiconductor device having its active region in this region can exhibit good characteristics. For example, it is possible to provide a semiconductor laser device in which the light emitting characteristics are less deteriorated and a field effect transistor having superior carrier traveling characteristics.
In the step (a), the etching mask may be a film made of a material etched during the etching in the step (b) and reduced in lateral size by the etching. In the step (a), also, the etching mask may be made of a material having translucency, and in the step (b), the size of the top surface of the convex portion may be reduced by etching a portion of the first semiconductor layer located below the etching mask while irradiating the first semiconductor layer with light from above the etching mask. In particular, in the step (b), the tilt angle of the side face of the convex portion may be controlled to be a desired value by selection of etching conditions.
The third method for fabricating a semiconductor device of the present invention includes the steps of: (a) forming a first semiconductor layer made of a Group III nitride on a substrate; (b) forming a second semiconductor layer made of a material having a function of adhering to the first semiconductor layer on the first semiconductor layer; (c) forming an etching mask on the second semiconductor layer; (d) etching the first and second semiconductor layers using the etching mask to form in the first semiconductor layer at least one convex portion having a top surface and a side face intersecting with the top surface; (e) forming a top epitaxial mask on the second semiconductor layer remaining on the top surface of the convex portion of the first semiconductor layer; (f) forming a third semiconductor layer on the first semiconductor layer by epitaxial growth after the step (e); and (g) forming a semiconductor element operating using a region of the third semiconductor layer located above the convex portion as an active region.
By the above method, basically the same effects as those obtained by the first fabrication method are obtained. In addition, this method can improve the adhesion between the top epitaxial mask and the first semiconductor layer, and thus improve the yield at the fabrication of the semiconductor device.
In the step (c), the etching mask may be formed by patterning a SiO2 film.
The step (c) may be performed by reactive ion etching with a gas containing chlorine.
The fourth method for fabricating a semiconductor device of the present invention includes the steps of: (a) forming an Al-containing underlying semiconductor layer and a first semiconductor layer made of a Group III nitride sequentially on a substrate; (b) forming an etching mask on the first semiconductor layer; (c) etching the first semiconductor layer using the etching mask to form in the first semiconductor layer at least one convex portion having a top surface and a side face intersecting with the top surface; (d) forming a top epitaxial mask on the top surface of the convex portion of the first semiconductor layer; (e) forming a second semiconductor layer on the first semiconductor layer by epitaxial growth after the step (d); and (f) forming a semiconductor element operating using a region of the second semiconductor layer located above the convex portion as an active region.
By the above method, basically the same effects as those obtained in the first fabrication method are obtained.
The method may further includes the step of forming a third semiconductor layer made of a material having a function of adhering to the first semiconductor layer on the first semiconductor layer after the step (a) and before the step (b). In the step (b), the etching mask may be formed on the third semiconductor layer. In the step (c), part of the third semiconductor layer may be left on the top surface of the convex portion, and in the step (d), the top epitaxial mask may be formed on the part of the third semiconductor layer. This permits improvement of the yield at the fabrication of the semiconductor device as described above.
When the third semiconductor layer is an AlAs layer, in the step (d), the top epitaxial mask can be formed by oxidizing a surface portion of the AlAs layer.
In the step (c), the first semiconductor layer may be etched until the underlying semiconductor layer is exposed to form the mesa-shaped convex portion on the underlying semiconductor layer, and the method may further includes the step of forming a bottom epitaxial mask by oxidizing an exposed surface portion of the underlying semiconductor layer.
The method for fabricating a semiconductor substrate of the present invention includes the steps of: (a) forming an etching mask on a first semiconductor layer made of a Group III nitride formed on a substrate for crystal growth; (b) etching the first semiconductor layer using the etching mask to form in the first semiconductor layer at least one convex portion having a top surface and a side face intersecting with the top surface; (c) forming a second semiconductor layer on the first semiconductor layer by epitaxial growth after the step (b); and (d) removing the substrate for crystal growth.
By the above method, it is possible to provide a semiconductor substrate suitable for fabrication of the semiconductor device of the present invention.
The method may further include the step of forming a top epitaxial mask used in the step (c) on the top surface of the convex portion of the first semiconductor layer.
When the substrate for crystal growth includes a base plate and an Al-containing underlying semiconductor layer formed on the base plate, in the step (b), the first semiconductor layer may be etched until the underlying semiconductor layer is exposed, and the top epitaxial mask may be formed by oxidizing an exposed surface portion of the underlying semiconductor layer before the step (c).